1. Field of the Invention
The present invention relates to a polyamide resin suitable for producing films, sheets, hollow molded articles such as bottles, and fibers. More specifically, the present invention relates to a polyamide resin produced by polycondensing a diamine component mainly comprising m-xylylenediamine and a dicarboxylic acid component mainly comprising adipic acid.
2. Description of the Prior Art
A polyamide produced by the polycondensation of a xylylenediamine and an aliphatic dicarboxylic acid, such as a polyamide produced from m-xylylenediamine and adipic acid (hereinafter may be referred to as “MX nylon”), has been used as a gas barrier material for films and bottles in the application fields of resin-reinforced injection materials and packaging materials, because of its high strength, high elastic modulus, low permeability to gaseous substance such as oxygen and carbon dioxide.
A polyamide resin has a terminal amino group and a terminal carboxyl group at its molecular chain ends. The terminal amino group and the terminal carboxylic group are subjected to amidation reaction to increase the molecular weight (increase in melt viscosity) when melted for producing shaped articles. The increase in the melt viscosity may result in a low flowing speed and a long-term retention of a molten resin in a molding machine. A resin thus heated for an excessively long period of time is subjected to not only further amidation but also cross-linking reaction to become three-dimensional, thereby likely to produce gelated mass that is different from a normal resin in their properties. In addition, the formation of gelated mass, etc. is accelerated if a molding machine has a portion stagnating a molten resin.
The formation of gelated mass may adversely affect the molding operation and the final products. Gelated mass formed on the inner wall of a flow path of molten resin in a molding machine causes uneven extrusion and uneven thickness of films and sheets. Gelated mass included in shaped articles creates defective portions such as fish eyes and hard spots to reduce the strength and spoil the appearance.
Therefore, to produce molded articles excellent in the strength and appearance by preventing the formation and inclusion of gelated mass, it is necessary to design a molding machine so as to minimize the stagnating portion, and additionally, it is inevitable to provide a high-quality polyamide which causes no abnormal reaction under melting.
One of the methods for preventing the amidation reaction under melting is to control the terminal amino concentration or the terminal carboxyl concentration of polyamide. “Kobunshi Ronbun-shu”, Vol. 37, No. 2, pp95-102 describes that the viscosity increase and the gelation of polyamide MXD6 under melting are prevented by using a starting adipic acid or m-xylylenediamine as a viscosity stabilizer, and that such effects are increased with increasing amount of the viscosity stabilizer. However, a large amount of the viscosity stabilizer breaks the molar balance between the monomers participated in the reaction to prolong the reaction time and fail to obtain a polyamide having a molecular weight enough to produce molded articles having a sufficient strength.
Japanese Patent Application Laid-Open No. 49-38950 proposes to add a phosphorus compound or an alkali compound to improve the heat stability of polyamides. However, as a result of measurement of the gelation initiating time in a manner described in the example portion below, the mere addition of the proposed compound is not sufficiently effective. In addition, since the alkali compound slows down the rate of polycondensation, the proposed addition amount leads to a prolonged reaction time and a large heat history, thereby making it difficult to prevent the gel formation in a stagnating molten polyamide.